Single-stage AC-DC power conversion is a low cost and thus popular power supply topology for applications such as solid state lighting. An important parameter for a single-stage AC-DC power switching converter is its power factor, which is the ratio of the real power delivered by the AC mains to the single-stage AC-DC switching power converter as compared to the apparent power delivered to the single-stage AC-DC switching power converter. The apparent power is insensitive to the phasing between the input current and voltage in contrast to the real power. The power factor is thus lowered if the input current and voltage are out of phase. The rectified input voltage to a single-stage AC-DC switching power converter cycles from approximately zero volts to the peak line voltage (e.g., 120 V*1.414 in the US) at twice the frequency for the AC mains. Given this sinusoidal pulsing or cycling of the rectified input voltage, the input current should have a similar profile to achieve a high power factor such as by the use of a suitably-modified peak current or constant on time control methodology.
To achieve high power factor in a single stage power converter, it is conventional to use either a peak current control methodology or a constant on-time control methodology. In both these techniques as shown in FIG. 1B, an input current 105 repeatedly pulses on and rises to a peak value before cycling off. Input current 105 may also be referred to as a magnetizing current. The peak value for input current 105 for each cycle has a profile or threshold envelope 100 that is similar to a rectified input voltage 110 for the switching power converter as shown in FIG. 1A so that a high power factor is achieved. But note that peak current profile 100 decreases down to zero at an AC input current zero crossing point 115. Should the input current be supplied by a triac as is conventional for dimming purposes in solid state lighting applications, this decrease in peak current profile 100 is problematic as a triac requires a minimum triac holding current threshold 120 to operate. The triac would thus reset as peak current profile 100 dips below minimum triac holding current threshold 120. The triac will continue to reset until peak current profile 100 again rises above minimum triac holding current threshold 120. The result of this multi-firing of the triac is that a user of a solid state lighting application such as an LED light powered by a high-power-factor single-stage power converter suffers from flicker. In addition, the switching power converter is subject to thermal stress.
To alleviate the flicker and thermal stress caused by multi-firing of the triac, it is conventional to implement a minimum peak current threshold 200 as shown in FIG. 2 for a peak current profile 205. Minimum peak current threshold 200 is chosen such that the resulting peak magnetizing current does not fall below minimum triac holding current threshold 120 discussed with regard to FIG. 1B. Although the triac is thus inhibited from resetting, the power factor is worsened as the rectified input voltage and peak current profiles then differ as the rectified input voltage drops below minimum peak current threshold 200. A tradeoff must thus be achieved between increasing the power factor and decreasing the likelihood of triac reset. Note that minimizing cost is critical in solid state lighting applications such that is conventional to eliminate the use of a snubber for the rectified input voltage. But the snubber elimination worsens the tradeoff as minimum peak current threshold 200 must typically be increased in snubber-less embodiments.
Accordingly, there is a need in the art for single-stage power converters having robust power factor correction while having sufficient triac holding current.